(1) Field of the Invention
The present invention relates to crimped fabrics which are formed by using various textile architecture such as woven, braided, knitted or other known fabric in which yarn families alternatively pass over and under each other and more particularly to the methods for producing a crimp-imbalanced fabric layer and a multi-layered fabric system having crimped imbalance gradients in the through-thickness direction for use in non-matrix reinforced fabrics and matrix reinforced fabrics. The category of matrix reinforced fabrics includes both flexible and rigid composites that utilize crimped fabrics.
(2) Description of the Prior Art
Crimped fabrics, such as a plain-woven construction example shown in FIG. 1, uniquely develop architectural changes on the meso-scale (yarn-to-yarn level) through crimp interchange as functions of biaxial tensions. Crimp interchange enables the tensile forces among yarn families to vary with applied multi-axial loading.
Crimp is typically defined as the waviness of a fiber or yarn in fabric form. Crimp interchange is the transfer of crimp content from one yarn direction to the other(s) as a consequence of fabric loading. Crimp interchange results from the relative motions of slip and rotation between yarn families at the yarn crossover points in response to applied loads. However, the extent of crimp interchange is generally more significant in non-matrix reinforced fabrics than in matrix reinforced fabrics. Crimp interchange is dependent upon the ratio of initial crimp content among yarn families and the ratio of stress between yarn families rather than the levels of stress alone.
Crimp interchange, which is a coupling mechanism analogous to Poisson's effect in traditional materials, produces substantial nonlinearities in the constitutive behavior of woven fabrics. These nonlinearities are generally less significant for matrix reinforced fabrics because the matrix limits the amount of yarn slip and rotation that occur at the yarn crossover regions. For example, stiff matrices such as metal, epoxy, vinyl ester, etc, will severely limit the relative yarn motions while compliant matrices such as rubber, urethane, etc. may allow appreciable relative yarn motions.
FIG. 2 and FIG. 3 identify crimp-related parameters in geometric models for plain-woven fabrics constructed of yarns of circular cross sections. The two types of crimping shown in the figures describe two possible cases for a plain-woven fabric. The parameters (recognizable to those ordinarily skilled in the art) for interpreting the use of the figures are: “d” is the yarn diameter (the same for the weft and warp yarns); “D” is the fabric thickness measured at cross-over points (overlap regions where the warp yarns cross the weft yarns); “p” is the distance between centers of adjacent yarns; “h” is the distance between centerlines of adjacent weft yarns (and h/2 is one-half of h, also note that when h=0, there is no crimp in the weft yarns); “alpha” is the crimp angle of the warp yarns; and “L/2” is one
quarter of the warp yarn's wave length shape (note that 4×L/2 equals one complete wave length of the warp yarn shape).
In the uni-directional crimp case depicted in FIG. 2, the yarns 2, 4 and 6 are not crimped. The yarns 2, 4 and 6 lie straight in the same horizontal plane and have zero waviness. Yarn 10 is crimped (having waviness) to allow placement amongst the other yarns 2, 4 and 6. Therefore, this type of fabric construction is said to be uni-directionally crimped—only one yarn family 10 has crimp content (i.e.: waviness). The bi-directional crimping of FIG. 3 depicts that both yarns families; that is yarns 2, 4, 6 and 10 have waviness (note that the yarns 2, 4 and 6 do not lie within the same horizontal plane—see reference line 12).
The parameters of FIG. 3 are applicable for defining the geometric dependencies of crimp in fabrics constructed with yarns or tows (non-twisted yarns) of alternative cross-sections. Many ballistic fabrics employ non-circular cross-section yarns such as rectangular, lenticular, elliptical, etc. Each type of yarn cross section provides slightly different sliding, interlocking, shearing and compaction compression (at the crossover points) characteristics at the points when the fabric is subject to extensional and shearing forces.
Crimp content is obtained by measuring the length of a yarn in a fabric state, Lfabric, and the length of the yarn after extraction from the fabric, Lyarn, and straightened out according to Equation (1).
                    C        =                                            L              yarn                        -                          L              fabric                                            L            fabric                                              (        1        )            
There exists a limiting phenomenon to crimp interchange. As the biaxial tensile loads continually increase, in a plain-woven fabric for example, a configuration results in which yarn kinematics (i.e.; slip at the crossover points) cease and the interstices (spaces) between converge to minimum values. This configuration is referred to as the extensional jamming point. The jamming point can prevent a family of yarns from straightening thus limiting stresses in those yarns and in extreme cases averting tensile failures. With the absence of failures in those yarns (for example: during a ballistic impact event) these yarns remain in position to provide a blunting mechanism that distributes the impact forces over a progressively larger number of yarns in subsequent fabric layers.
Research investigating ballistic impact mechanics of crimped fabrics has recognized the role of crimp interchange. Crimp interchange is often explored together with inter-yarn friction mechanisms because both involve sliding interfaces among yarn surfaces at the crossover points.
Research in woven ballistic fabrics has produced findings that: (1) generally purport ranges of desirable friction coefficients for optimal ballistic protection performance measured in terms of a V50 designation; (2) identify limiting bounds of these coefficients for use in numerical and analytical models; and (3) establish the need for sizing methods to affect fiber roughness. Ballistic protection limits are designated by V50, which is the velocity at which an armor panel of a given areal density has a 50% probability of stopping the projectile at zero degree obliquity.
Crimp effects in structural fabrics have also been researched. In the area of pneumatic structures, air beams were researched to establish the combined biaxial and shear behavior of plain-woven fabrics, non-matrix reinforced fabrics. Both meso-scale unit cell and fabric strip models were validated. The results indicated that crimp interchange, decrimping and shearing (also referred to as trellising—FIG. 4, FIG. 5 and FIG. 6) play major roles in the mechanical response of crimped fabrics subjected to applied structural forces. FIG. 4 depicts an unloaded state of woven fabric; FIG. 5 depicts a shearing (trellising) state of woven fabric and FIG. 6 depicts a shear jamming stage of woven fabric.
Shear trellising and shear jamming are the terms given to the configuration of a fabric subjected to pure shear. Consider the lower ends of the vertical yarns 20 clamped and the right ends of the horizontal yarn 22 clamped. Now, consider a horizontal force applied to the upper end of the vertical yarns 20. This is the shearing mode of loading that will cause the yarn rotations (trellising) and eventual yarn jamming states.
The advantageous effects of functionally grading crimp imbalance along the through thickness direction of multiple layered fabric systems by design on soft fabric armors and matrix reinforced fabrics have not been sufficiently explored as a mechanism for increasing performance attributes such as ballistic, penetration, blast and shock protection levels as well as flexibility.
In the prior art, U.S. Pat. Nos. 6,720,277; 6,693,052; 6,548,430; 5,976,996; 5,837,623; and 5,565,264 relate to fabric substrates of woven constructions having principally two yarns, namely warp and fill (also referred to as weft), aligned in an orthogonal layout in accordance with a plain-woven architecture. These cited references claim a variation of crimp contents between the warp and weft yarn directions within a single woven fabric layer but do not achieve the improved performance attributes obtained when bias yarns are added within a plain-woven fabric and thus creating a three-dimensional woven fabric. The addition of bias yarns within a woven fabric will
reduce regions of oblique susceptibility caused when penetrators impact the fabric to enhance protection levels.
While the cited references again claim a variation of crimp contents between the warp and the weft yarn directions within a single woven fabric; the present invention describes a system of multiple crimp-imbalanced layers arranged such that the levels of crimp imbalance vary among the layers in the through thickness direction to enable functionally graded performance attributes such as enhanced ballistic, stab, blast and shock protection levels which can improve strength and damage tolerance levels and reduce blunt trauma in personnel protection systems.
Furthermore, the cited references of Howland describe plain-woven fabrics possessing cover factors (CF) up to one hundred percent for warp fibers at the weft center and in excess of seventy-five percent for the weft. It has been defined in the art that a cover factor on the geometrical sense as the fraction of orthogonally-projected fabric area that is occupied by yarns. As the cover factor increases so does stab penetration protection because the interstices between yarns decrease in size, which increases the resistance of the yarns to be pushed aside by sharp pointed penetrators.
Highly dense, tightly woven fabrics are required to defeat punctures from stab impacts. However, this type of construction performs poorly during ballistic impact because the yarn motions are severely restricted. Past experience has demonstrated that multi-threat armors, also referred to as “in-conjunction armors” designed for combined ballistic and stab protections were essentially produced with two component armors: one for ballistic protection and one for stab protection. Fabric design requirements for ballistic versus stab protection are often antagonistic. Accordingly, crimp-imbalanced woven fabric architectures have the capacity to simultaneously increase both stab and ballistic resistance.
Technology advances in soft fabric armor designs have focused on two principal construction methods (layered woven armor systems and uni-directional, cross-ply, layered armor systems). FIG. 7 depicts uni-directional layers arranged in multiple 0/90 degree stacks. The uni-directional layers are often adhered to form the stacks by using compliant binder films that act as a matrix to provide minimal reinforcement to the stacks. During a ballistic impact, the uni-directional yarns dissipate the kinetic energy rapidly due to the absence of yarn crossover points. The crossover points in woven fabric armors reflect portions of the stress waves back to the impact zone rather than entirely transmit the waves away from the impact zone. These reflections reduce the amount of energy absorbed by crimped fabrics.
A disadvantage of uni-directional constructed fabric armor is the trade in comfort and flexibility for the incremental increase in ballistic protection. While this does not present a usability issue for vehicle and structural armor, it can be an issue for flexible (soft) body armor. This is because uni-directional fabric armors are not interlaced; that is, no yarn crossover points exist to enable the relative motions among yarn families that produce flexibility and conformity.
A need therefore exists for technological advances in single and multiple ply crimped fabric architectures and therefore advances in both non-matrix reinforced and matrix reinforced fabric systems for use in protective fabrics, fabric structures and composite structures.